14. New Via Amendment A00002 - Los Alamos Creek Bridge Geotechnical Evaluation.pdf

Cleveland National Forest

Proposed Los Alamos Creek Bridge Geotechnical Site Evaluation

Prepared by Michelle L. H. Bearmar, PE

SoCal Province Geotechnical Engineer For Tammie Mather, Project Engineer Cleveland National Forest, Region 5

Report Contents

1.0 INTRODUCTION

2.0 GEOLOGY AND GEOTECHNICAL

2.1 GEOLOGY

2.2 ROCK AND SOIL

2.2.1 Site Visit

3.0 RECOMMENDATIONS

3.1 BEARING CAPACITY

3.2 SETTLEMENT

3.3 SCOUR POTENTIAL AND PROTECTION

3.4 BACKFILL AND COMPACTION

3.4.1 Approach Roads

3.4.1.1 Subgrade and Road Base

3.4.1.2 Embankment Fill

3.4.2 Abutment and Piers

3.4.2.1 Foundations

3.4.2.2 Abutment backfill

4.0 CONCLUSIONS

5.0 REFERENCES

APPENDIX A - MAPS

A.1 LOCATION MAP

A.2 VICINITY MAP

A.3 PROJECT AREA MAP

A.4 AERIAL VIEWS

A.4.1 Project Vicinity A.4.2 Project Area

A.4 LOCAL GEOLOGY MAP

A.5 SITE PLAN

APPENDIX B – BEARING CAPACITY GRAPHS

B.1 ABUTMENTS

B.1.1 Shallow

B.1 ABUTMENTS

B.1.2 Deep

B.2 PIERS

B.2.1 Shallow

B.2 PIERS

B.2.2 Deep

APPENDIX C – SETTLEMENT GRAPHS

C.1 ABUTMENTS

C.1.1 Shallow

C.1 ABUTMENTS

C.1.2 Deep

C.2 PIERS

C.2.1 Shallow

C.2 PIERS

C.2.2Deep

APPENDIX D – SCOUR CALCULATIONS

D.1 CHANNEL PROPERTIES

D.1.1 At Bridge Site

D.1 CHANNEL PROPERTIES

D.1.2 Upstream 170 feet

D.2 CONTRACTION SCOUR

D.3 CRITICAL VELOCITY FOR D50 MATERIAL

D.3.1 At Bridge

D.3 CRITICAL VELOCITY FOR D50 MATERIAL

D.3.2 Upstream 170 feet

D.4 LOCAL PIER SCOUR

1.0 Introduction

The Cleveland National Forest is proposing to replace an existing concrete low water crossing or ford on Los Alamos road with a new triple-span concrete bridge. This bridge will span the channel in Los Alamos Canyon with two piers in the 100-year flood plain and be capable of providing crossing to vehicular traffic during the 100 year storm event. Removal of the existing low-water crossing and installation of the bridge will potentially facilitate aquatic passage between the upstream reaches of Los Alamos Canyon and the San Mateo Creek further down.

Los Alamos Road is located in Riverside County on the Trabuco Ranger District. The proposed bridge site is approximately ¾-mile from the intersection of Los Alamos with Tenaja Road and just upstream of the confluence of Los Alamos Creek with Wild Horse Canyon, a rather large tributary that extends north and up to Elsinore Peak.

The purpose of this report is to provide information to the multi-disciplinary team which will assist in decisions associated with project design. It is limited in scope to geology and geotechnical issues as well as fluvial concerns related to channel scour at the structure and deposition near the structure.

2.0 Geology and Geotechnical

2.1 Geology

The area containing the concrete low-water crossing and proposed bridge site is located in Los Alamos Canyon, a major tributary to the southwesterly draining San Mateo Canyon in the Elsinore Mountains. These mountains are located at the southern tip and are a sub-range of the Santa Ana Mountains which lie within the Peninsular Ranges Geomorphic Province. The highest, northern-most peaks in the range are composed of Triassic-Jurassic Period metasedimentary rocks (about 225 million years old). These formations contain the oldest exposed rocks in Orange and eastern Riverside County. Remnants of these formations exist to the south and can be seen in Los Alamos Canyon.

During Jurassic-Cretaceous era (about 170 m.y.a.) the basic and ultra-basic crust of the Pacific Plate was plunging beneath the continental margin and some of this melt made its way upward as intrusive plutons. These rock bodies tend to be more resistant to erosion than most others and can be found scattered throughout the southern end of the Santa Ana Mountains. Together with the Santiago Peak Volcanics, which overlaid the intrusive plutons and the metasediments alike about 150 m.y.a., these remnants form many of the plateaus found in San Diego and Riverside Counties.

The Santa Rosa Plateau is one such feature which is located to the south of and drains into Los Alamos Canyon.

Subduction and melting of the existing continental margins continued for the next 70 million years creating large magma chambers along the west coast of present day California and Oregon.

The Peninsular Ranges Batholith (and geomorphic province), which stretches from the Los Angeles Basin down into Baja California, is a result of the cooling of these of the subduction related activities. This granitic core forms much of the basement rock in the western portion of southern California south of the Los Angeles basin. This is the granitic “bedrock” material seen in Los Alamos Canyon at present day.

Much of the rest of the area’s history contains deposition of marine sediments as the coastline was underwent periods of uplift and then reclamation by the coastal shallow seas.

Then around 5.5 million years ago the area became once again very interesting. As oceanic spreading center merged into the subduction zone, the Farallon Plate disappeared beneath the continental plate known as the North American Plate and the boundary between the Pacific Plate and the North American Plate shifted down to the present day location of the Gulf of California giving rise to the San Andreas Fault Zone. An oceanic spreading center opened up to the south ripping the finger of Baja California from its mainland Mexico. Consequently the San Andreas and a multitude of associated faults appeared tilting, folding and fracturing the landscape as the Santa Ana and surrounding mountain ranges rose to their existing elevations.

The area in the vicinity of the crossing is dominated by granitic rocks of the early cretaceous. These appear to be predominately granodiorites and tonalites. Additionally Jurassic marine meta-sediments are shown as present within the project area (Corps of Engineers, Army Map Service, 1965, sixth printing 1992).

2.2 Rock and Soil

2.2.1 Site Visit

A site visit was performed on Wednesday, 11 April with Regional Office Bridge Team Designers Nancy Tipton and Jon Sampson, Shasta Trinity National Forest Roads Engineer Ken Kellogg, Hydrologists Kyle Wright from Sequioa National Forest and Mikaila Mimbenieks from San Bernardino National Forest, along with Cleveland National Forest Engineers Steve Eastwood and Tammie Mather. Two potential alignments were examined along with the general hydrology, soils and geology.

The primary alignment crosses the stream below the existing concrete low-water crossing (LWC) and takes off from the main road near a pullout, swinging close to the south edge of the road and the rock face, followed by a curve to the north, crossing Los Alamos approximately where the LWC starts and finishing approximately 50 feet downstream of the distal end of the crossing.

The additional alignment curves south and crosses Los Alamos between 40 and 100 feet upstream of the LWC.

The slope on the west side of Los Alamos near the LWC is characterized by jointed granitic rock, probably diorite or granodiorite. This rock is covered with a thin layer of regolith soil and light vegetation and appears to be fairly massive. Rippability will be related to jointing and weathering. Small amounts of rock slope removal may be done with heavy equipment. Extensive movement into the slope will likely require some blasting techniques. This granitic rock appears to continue to the east side of the stream just south of the crossing. Bedrock in the stream channel and an outcrop seen in the road cut on the east side confirm this.

Downstream of the LWC the substrate is generally coarse sand mixed with about 50 percent cobble and boulder, mostly tonalite from the hills to the north. An aerial view shows this area to be part of a larger wedge that appears to be the terminal end of a large braided channel which deposits alluvial/colluvial material from Wild Horse Canyon, a 4-mile long drainage which extends north to Elsinore Peak. Additionally a short drainage, approximately ½-mile in length, joins this confluence just to the east and below the LWC probably enhancing the width of the fan spilling into the main channel.

3.0 Recommendations

3.1 Bearing Capacity

No in-situ or laboratory sampling was performed at abutment or pier locations. Granular material in the approximate proposed location of the pier appears to be of coarse gravelly sand while the abutment location (on the west end) is a silty sand. Assumptions about soil conditions are made based on field observations and experience but are conservative. Field verification of subgrade footing conditions needs to be verified before construction.

Bearing capacities for the abutments and center piers are calculated from Terzaghi’s equations for continuous footings and also for rectangular footings, all using various widths and depths. The continuous footings are included to allow the designer to look at a deeper foundation (where the depth to the bottom of the footing is at least greater than the width of the footing) whereas the shallow rectangular footings are for the purpose of designing a foundation where the depth to the footing base does not exceed the width of the footing. These capacities are incorporated into graphs which allow the designer to choose a size and type which best suits the situation for each structure. It was assumed that depth of groundwater could exist at the base of the footing for abutments as well as piers during design high flows; as a result effective unit weight was used in these formulas. A factor of safety of 3.0 has been applied to produce the final values. Additionally ultimate load values for LFRD are available as graphs in Appendix B.

3.2 Settlement

Predicted short-term settlement on granular soils is based size and depth of footing, presence of water, and modulus of vertical subgrade reaction. Settlements here are based on total allowable bearing capacity for footing size and depth. Use of a smaller design load should yield a reduction in actual short-term settlement. Larger graphs are incorporated into Appendix C.

3.3 Scour Potential and Protection

Based on information taken from the Preliminary Layout AutoCAD drawing, supplied by the bridge group out of the Forest Service Region 5 Regional Office, a pre-burn peak flow of 6,000 cfs can be expected during the 100-yr design storm. The current bed profile under the initial preliminary proposed bridge alignment with 2 in channel piers has an area of 661 square feet and a 144-foot wetted perimeter at high water. Using a roughness of 0.068 taken from the bridge group data and a slope of 1.4% (from the AutoCAD drawing), this equates to a velocity of 9.1 ft/s and a non-scoured depth of 9.88 feet at the site. From the hydrology report titled San Mateo Creek Write Up (Kwan & Wright, 2011) the D50 in the streambed just downstream of the proposed bridge site is quite large, on the order of approximately 6 inches. Using the D50 of 6-inches the critical velocity required for incipient motion of material 6 inches and below is 13 fps which is well above the design storm velocity. At 9.1 fps it is anticipated that material below 2.05 inches (52mm) will mobilize which equates to approximately 25 percent of the material near the crossing.

Based on upstream flow and mobile bed material, general scour concerns are related to clearwater scour. The channelized flow will be slightly contracted relative what it is currently, but as a result of the size of the mobile material and the amount of contracted flow, contraction scour should not be an issue even with two 3-foot wide piers in the channel. Assuming a rectangular center pier 3-feet wide, pier scour would be approximately 5 feet at the anticipated 100-year flow velocity.

This depth can be reduced slightly by an alternative nose shape on the pier.

Rip rap sizing should have a D50 of approximately 334 pounds (1.56 ft diameter) or a Class 5 rip rap based on Table 705-1 of the 2003 Forest Service Specifications for Roads and Bridges (FP- 03).

Long-term lowering of the bed should also be accounted for. If removal of the existing concrete low-water crossing is to take place then several feet of gradient adjustment could take place in the vicinity of the bridge. This adjustment will most likely happen over time with the progression of head cutting upstream probably during high flow events.

3.4 Backfill and Compaction

No in-situ sampling or laboratory sampling of site material was performed. All conclusions are based on visual observations and experience. Soil material in the area of the west abutment appears to be of silty sand with a mixture of cobbles and some boulders. The material may be useful as embankment fill if deleterious material is removed and rock greater than 6 inches is screened out.

3.4.1 Approach Roads

3.4.1.1 Subgrade and Road Base

It is suggested that prior to paving, testing of in-situ subgrade material in the road bed alignment be performed for both the east and west approaches in order to determine suitability of in-situ material for paving and for compaction requirements. Tests should include at a minimum gradation analysis (AASHTO T-27 & T-11) and R-Value (ASTM 2844) tests for paving suitability and maximum dry density with optimum moisture content (ASTM D 1557) for compaction. Scarify the road bed to a minimum depth of 12 inches removing all deleterious material and material greater than 2 inches. Subgrade should conform to the following gradation requirements (or as approved by Forest Service geotechnical engineer):

Percent by Mass Passing Designated Sieve Sieve Size (AASHTO T 27 & T 11) 2 inch 100 No.4 30-70 No. 200 0-10

Compact subgrade to 90 percent of maximum dry density (MDD) within the optimum moisture range (OMR, to be determined after testing). If no base material is required, compact to 95 percent of MDD. After an R-Value is determined for in-situ subgrade material, if needed the required amount of aggregate base meeting the specifications of Caltrans Class II Base, ¾-Inch Max. can be determined. Compact the upper 12 inches of base/subgrade material to 95 percent of MDD at OMR.

3.4.1.2 Embankment Fill

The upper 2 feet of any embankment shall consist of non-deleterious material 3 inches or less in diameter and have the following gradation requirements (or as approved by Forest Service geotechnical engineer):

Percent by Mass Passing Designated Sieve Sieve Size (AASHTO T 27 & T 11) 3 inch 100 1 inch 70-100 No. 4 30-70 No. 200 0-10

Embankment fill other than the upper 2 feet shall consist of non-deleterious material 6 inches or less in diameter and have the following gradation requirements (or as approved by Forest Service geotechnical engineer):

Percent by Mass Passing Designated Sieve Sieve Size (AASHTO T 27 & T 11) 6 inch 100 No. 40 50 max No. 200 25 max,

3.4.2 Abutment and Piers

3.4.2.1 Foundations

In-situ soil in the location of the west abutment should make a good foundation material for that bridge abutment however Forest Service needs to verify field conditions at bottom of footing prior to construction. Dewater pier and abutment locations as needed during site preparation and construction of footings. Remove unsuitable material and replace with non-deleterious foundation material 2 inches or less in diameter with a Plasticity Index of 6 or less and meeting the following gradation requirements (or as approved by Forest Service geotechnical engineer):

Percent by Mass Passing Designated Sieve Sieve Size (AASHTO T 27 & T 11) 2 inch 100 No. 10 50 max No. 40 30 max, No. 200 6 max

Compact the upper 12 inches of imported foundation material to 95 percent MDD and the remainder to 90 percent MDD.

Upon approval of in-situ subgrade material, if applicable, obtain soil test results required to ensure compaction performance. Tests should include at a minimum gradation analysis (AASHTO T-27 & T-11) and maximum dry density with optimum moisture content (ASTM D 1557) for compaction. Scarify the foundation to a minimum depth of 12 inches (or as directed by Forest Service technical representative) removing all deleterious material and material greater than 2 inches. Compact to 95 percent MDD within OMR.

Bearing capacities for various footing arrangements are given in the previous section under Bearing Capacities.

3.4.2.2 Abutment backfill

Backfill material within 5 feet of abutments shall be a non-deleterious free draining material less than 4 inches in diameter and meeting the following gradation requirements (or as approved by Forest Service geotechnical engineer):

Percent by Mass Passing Designated Sieve Sieve Size (AASHTO T 27 & T 11) 4 inch 100 No. 40 60 max No. 200 6 max

Compact the upper 12 inches of imported foundation material to 95 percent MDD and the remainder to 90 percent MDD.

4.0 Conclusions

Information provided in this report is more of a cursory nature based on site observations during two separate visits. A more detailed geotechnical investigation might include exploratory borings along with in-situ and laboratory tests to determine stratigraphic soils information, elevation to bedrock, and soil strength parameters. Much of that type of information which is presented here is either estimated or correlated based on topographical observations made on site and using geologic maps, historical data and on-site field soil classifications. The design values presented in this report are at the lower end for the soil types and almost certainly conservative under the circumstances. As this structure is for a low traffic volume road and not a critical or high hazard structure, the additional cost and time of a more extensive site investigation may not be necessary. More diligence in inspection of actual subsurface conditions will be required during the construction phase however.

A three-span structure with two in channel piers and armored abutments is feasible from a geotechnical standpoint. Careful consideration should be given to sighting the structure based on channel morphology and the potential debris flow from Wild Horse Canyon. Scour depth and protection are important concerns in the location of the center pier at the proposed location downstream of the existing concrete crossing. Given these two important factors the alternative upstream location considered during the site visit may be more favorable from a geotechnical and geologic standpoint. However it is recognized that other factors including road alignment, geometry and proximity to wilderness boundaries may dictate a higher level of acceptable risk. If a definitive site is selected a backhoe excavation may be useful, depending on location, for determination of depth to bedrock.

The appendix to this report includes important site information, maps and backup data used to make recommendations in this report. Please feel free to contact me with questions regarding this information or any information contained within the report.

5.0 References

Coduto, D. P. (1994) Foundation Design, Principles and Practices, Prentice Hall, Inglewood Cliffs, NJ, 796 pp.

Coduto, D. P. (1994) Geotechnical Engineering, Principles and Practices, Prentice Hall, Upper Saddle Cliffs, NJ, 759 pp.

Corps of Engineers, Army Map Service. (1965, sixth printing 1992). Topographic Base Map, Santa Ana Quadrangle,250,000 scale. Geologic Map of California, Olaf P. Jenkins Edition, Santa Ana Sheet. USA: USGS.

Department of the Navy (1982) Foundations and Earth Structures, Design Manual 7.2, US Government Printing Office, Washington D.C., 253 pp.

Department of the Navy (1982) Soil Mechanics, Design Manual 7.1, US Government Printing Office, Washington D.C., 364 pp.

Kwan, H., & Wright, K. (2011). San Mateo Creek Write Up. Sequoia National Forest.

LMNO Engineering, Research, and Software, Ltd. (n.d.). Manning's n Coefficients for Open Channel Flow. Retrieved May 2012, from The fluid mechanics calculations website:

http://www.lmnoeng.com/manningn.htm

US Department of Transportation, Federal Highways Administration, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-03), Washington D.C., USGS. (n.d.). Verified Roughness Characteristics of Natural Channels. Retrieved May 2012, from

SURFACE-WATER FIELD TECHNIQUES:

http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/index.htm

Michelle L. H. Bearmar SoCal Province Geotechnical Engineer, USFS Civil Engineer, PE

Appendix A

Maps

A.1 Location Map

A.2 Vicinity Map

A.3 Project Area Map

Project Location UTMs Datum: WGS84 11S 464349mE, 3712424mN

A.4 Aerial Views

A.4.1 Project Vicinity

A.4.2 Project Area

A.4 Local Geology Map

A.5 Site Plan

Appendix B

Bearing Capacity Graphs

B.1.1Shallow B.1 Abutments

B.1.2Deep B.1 Abutments

B.2.1 Shallow B.2 Piers

B.2.2 Deep B.2 Piers

Appendix C

Settlement Graphs

C.1.1 Shallow C.1 Abutments

C.1.2 Deep C.1 Abutments

C.2.1 Shallow C.2 Piers

C.2.2Deep C.2 Piers

Appendix D

Scour Calculations

D.1.1 At Bridge Site D.1 Channel Properties

D.1.2 Upstream 170 feet D.1 Channel Properties

D.2 Contraction Scour

D.3.1 At Bridge D.3 Critical Velocity for D50 Material

D.3.2 Upstream 170 feet D.3 Critical Velocity for D50 Material

D.4 Local Pier Scour